1. Field of the Invention
The present invention pertains to the art of forming lipid bilayer membranes and, more particularly, to forming planar lipid bilayers across small orifices for use in ion channel sensing systems.
2. Discussion of the Prior Art
A planar lipid bilayer (PLB), also referred to as a black lipid membrane (BLM), is a membrane composed of lipids that spans an orifice in a synthetic support. PLBs are used extensively as a controlled environment for the study of ion channel function. Once a PLB is formed, ion channels can be inserted into the PLB by either the spontaneous insertion of water soluble ion channels from bulk solution or by the incorporation of ion channels by fusion of ion channel containing liposomes. Ion channels are membrane proteins that play a critical role in a vast number of physiological processes including neural signaling, muscle excitation, hormone secretion, blood pressure regulation, electrolyte balance and cell death. Due to their importance in such a broad range of physiological functions, the detailed function of ion channels, as well as their interaction with drugs and other chemicals, is a field of intensive scientific research. Ion channels are often studied in their natural cellular environment, but the PLB provides a far more controlled and accessible means to study the behavior of a wide range of ion channels.
Ion channels present in lipid bilayers are also useful as components of biosensors. The presence of specific chemicals and changes in environmental conditions, such as pH, electrolyte concentration, and temperature, affect the function of the ion channel. By monitoring the conductance of an ion channel, it is possible to detect the presence of a wide variety of chemical agents.
Nanometer and micron scale apertures, referred to as “nanopores”, provide a robust platform for the formation of lipid bilayers and measurement of ion channel currents. The small area of bilayers formed on nanopores leads to long bilayer lifetimes, insensitivity to vibrations and a very low capacitance across the bilayer. The vibration insensitivity reduces the noise level of the ion channel measurement by eliminating coherent noise sources from the environment surrounding the apparatus. The low area of the bilayer minimizes current noise due to the conductance of time varying currents across the bilayer itself. Nanopores produced in glass and quartz substrates have an added advantage that the high resistance and low dielectric of the material further reduce the current noise of the ion channel measurement.
The lipids in a PLB are typically phospholipids, but bilayers may also be formed from a wide variety of lipids including glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, prenol lipids, fats, fat acyls, sterols and polyketides. Typically, PLBs are formed across a single aperture (e.g., 10-200 μm in diameter) in a hydrophobic film (e.g., Delrin™, polystyrene, polysulfone, polymethyl methacrylate), or over a drawn glass patch pipette (e.g., 100 nm to 5 μm in diameter). A PLB can also be formed over a glass nanopore membrane (GNM), which is comprised of a single conical-shaped pore (10 nm to 2 μm radius orifice) embedded in a thin glass membrane (˜50 μm), at the end of a cyano-silane coated glass capillary. PLBs have also been formed across apertures in silicon, silicon nitride and silicon oxide deposited on a semiconductor surface. The surface properties of the substrate utilized have a dramatic impact on the structure of the bilayer.
Currently, there are a number of methods of PLB formation known to those familiar with the art. These include: apposition, painting, tip dipping and microfluidics, which will be discussed in more detail below. In the apposition method, a hydrophobic film containing a single aperture is mounted between two compartments of a cell. The two compartments are then filled with an electrolyte solution so that the heights of the solutions are below the aperture. Next, the hydrophobic film is pretreated by placing a drop of hexadecane on the surface of the film in the vicinity of the aperture. A mixture of lipid/pentane is placed drop-wise on the surface of both electrolyte solutions and the pentane is given time to evaporate, leaving lipid monolayers on the surface of the electrolyte solutions. One electrolyte solution is then raised up past the aperture, depositing a lipid monolayer on the surface of the hydrophobic film and one leaflet of the bilayer that will span the aperture. The other electrolyte solution is then raised up past the aperture, depositing a lipid monolayer on the opposite side of the hydrophobic film, and the second leaflet of the bilayer which spans the aperture. This technique is well known to form solvent free bilayers, i.e., the PLB does not contain a significant amount of organic solvent. However, apposition cannot be utilized in systems wherein only one side of an aperture is accessible, e.g., a glass nanopore membrane or an array of nanopores with individual readouts.
The tip dip method is commonly used to place planar lipid bilayers over patch pipettes and involves first placing a patch pipette tip into a cell containing an electrolyte solution. A lipid/organic solvent mixture is then placed drop-wise on top of the electrolyte solution and is allowed to spread out, forming a lipid monolayer across the surface. Next, the patch pipette tip is pulled out of the electrolyte solution, which deposits a lipid monolayer on the surface of the pipette tip, with the hydrophilic head groups oriented towards the water layer on the glass surface. The pipette tip is then pushed back through the lipid monolayer at the air/water interface to deposit the second lipid monolayer onto the already existing monolayer and form a lipid bilayer. These lipid bilayers are known to contain a small fraction of organic solvent. Since the surface of the patch pipette is hydrophilic, the PLB extends past the orifice and around the side of the patch pipette. This extended bilayer allows ion channels to occupy the supported lipid bilayer on the glass in a region not over the aperture itself. Ion channels in this region will have a tendency to produce anomalously low conductances due to the high impedance of the water layer between the supported lipid bilayer and the glass.
The painting method involves the spreading of lipid/organic solvent across the surface of a hydrophobic film, using a pipette tip, in order to deposit a lipid bilayer across an aperture in that film. In the painting method, the surface of the film is initially “primed” with lipid/organic solvent. Typically, priming constitutes utilizing a pipette tip to deposit lipid/organic solvent onto the hydrophobic film. This lipid/solvent is then lightly spread out across the surface of the film around the aperture and allowed to dry. After drying, the film is placed in a two compartment cell that is separated by the single aperture. Both sides of the cell are then filled with electrolyte solution so that the height of each solution lies above the aperture. The film is then primed a second time under solution. A clean pipette tip is then used to drag the lipid/solvent on the surface of the film across the aperture, spontaneously forming a lipid bilayer. The painting method is a manual process that requires considerable skill to accomplish and does not lend itself to automation. Additionally, the PLB formed by painting contains a large amount of solvent due to the nature of the process. High solvent levels in the PLB are known to affect the function of incorporated ion channels.
PLBs have also been formed in microfluidics devices. With this method, for instance, a 100 μm diameter aperture is created between an upper chamber and a 0.5 mm high by 2 mm wide channel. An aqueous electrolyte is added to the upper chamber and a lipid/solvent solution is flowed through the channel past the aperture. Air is passed through the channel to remove the lipid/solvent solution and an aqueous electrolyte is introduced to the channel. The outer chamber is sealed, a pressure is applied, and the PLB spontaneously forms across the aperture. The microfluidic PLB formation method requires a large number of steps and has not been shown to work for apertures under 100 μm in diameter.
The above-discussed bilayer formation methods either require significant training (e.g., painting and tip dip), large pore sizes, many steps or require open access to both sides of the aperture across which the bilayer is formed. Currently, there is no available method to reliably and automatically form lipid bilayers across an aperture when only one side of the aperture is accessible. Accordingly, there is seen to be a need in the art for a method and system in which lipid bilayers can easily and reproducibly be directly deposited even if access is restricted to only one side of the aperture.